3' Turbo Back Exhaust

I have a a3 1.8t, agu engine, am going to be getting remap, forge tip, panel filter, fmic, forge 007p valve and later on maybe bigger turbo, as i have my own fabrication business am going to make myself my own system i already make the downpipes for the a3 & seat ibiza cupra but i would like to know if i can get away with a 3' downpipe de-cat with full 3' system with a large middle box and a small back box or would i be better with 2.5? am hoping to have about 300bhp later on in the future

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